1. Field of the Invention
This invention relates to an improvement in the techniques of forming flat glass and more precisely of those techniques according to which the glass sheets are shaped by application against a bending mold with forces of a pneumatic nature.
2. Background of the Prior Art
Numerous processes are known of forming a glass sheet according to which the glass sheet is loaded in horizontal position into a reheating furnace intended to bring its temperature to above 500.degree.-600.degree. C., in which it is conveyed on a conveyor such as a roller bed which extends downstream to a bending station. In this latter the sheet is taken over by a mobile transfer device at least in a vertical direction, then placed on a recovery frame which then brings it to the tempering station or to any other cooling station.
Depending on the case, the forming takes place either at the time and after the placing of the glass sheet on the recovery frame which then advantageously consists of an open ring whose contour corresponds to the contour it is desired to impart to the glass sheet, the bending then being performed under the effect of the forces of gravity and inertia, or at the time of taking over of the glass sheet by the transfer element which then comprises an upper bending mold whose curvature corresponds to that of the shaped glass sheet, or also by a combination of the two cases cited above.
The forming processes by application of the glass sheet against the upper bending mold are particularly advantageous, because they allow a better control of the deformation of the glass in its central part, while perfectly meeting the shape desired for the glass contour.
Such a process is described in the publication of French patent FR No. 2 085 464. According to this document, the glass sheet is conveyed into a reheating furnace, in horizontal position, by a conveyor of the roller bed type which extends downstream from the furnace to a bending station. There the glass sheet is immobilized then transferred vertically by a suction due to a low pressure created around the periphery of the sheet, to a curved, nonperforated upper bending mold against which it is applied to be shaped, according to the desired main curvature. The low pressure is obtained by placing the upper bending mold in a box without bottom or skirt, connected to suction means, and whose inside contour is slightly greater than that of the glass sheet while the contour of the upper bending mold is slightly smaller than the latter. Optionally, a complementary curvature is then given to the glass, for example, by pressing.
The main drawback of this type of device comes from the fact that the dimensions and geometry of the upper mold and of the bottomless box are rigorously controlled by those of the glass sheet itself. This means that any modification relating to the glass sheet involves the necessity of simultaneously replacing the box and the upper bending mold.
This replacement must also be performed frequently if the glazings are at least partially enameled. In this case, the enamel at times has a tendency to stick to the covering of the upper bending mold, a covering of refractory paper or fabric glued to the mold with refractory cement. The enamel then causes a rapid deterioration of this covering and the upper mold has to be withdrawn from the bending station, allowed to cool, and a new covering glued in before being able to use it again.
Another limit of this process comes from the fact that to shape glass sheets in a small radius of curvature, considerable suction powers are necessary, the distance between the upper bending mold and the box then being increased because of the curvature of the mold. In practice, this limits the curvatures that can be obtained with this type of forming.
Further, to obtain a sufficient power of suction of the glass sheets, it is necessary that the free space left between the sheet and the side walls of the box remain small. Also, with the slightest offset of the stop position of the glass, the latter hits the side walls of the box at the time of its movement and it must then be rejected.